1. Field of the Invention
The present invention relates to a method and apparatus for multiplexing broad band high frequency signals in general and in particular to a method and apparatus for multiplexing broad band high frequency signals in a network analyzer.
2. Description of the Prior Art
A network analyzer is an apparatus used for analyzing the electrical characteristics and performance of electrical networks and components thereof at various frequencies. In the analyzer, a network or component being analyzed is called a device under test or DUT. The signals transmitted through the DUT and/or reflected from the DUT are measured against a reference signal. The resulting ratios, i.e. S parameters, are a measure of the DUT's performance over the frequency range(s) of the signals applied thereto. The frequencies applied to a DUT may range from DC to over 60 gigaHertz 60 GHz depending on the type of network or component being analyzed.
To facilitate the testing of broad band devices, signals over a wide range of frequencies are typically generated by means of a plurality of signal generators, each of which provides a signal having a predetermined range of frequencies, e.g. 0.01-8 GHz, 8-20 GHz, 20-26 GHz, etc. When a plurality of such signal generators is used, the outputs of the signal generators are multiplexed, i.e. combined. Heretofore, it has been the practice to combine the outputs of the signal generators by means of a resistor network or some type of multithrow switch, e.g. an electro-mechanical or an electronic (PIN or FET) diode switch. However, each of these means has certain advantages and disadvantages.
An electro-mechanical switch has the advantage of producing low losses at high frequencies, but has the disadvantage of having a limited switching life and, therefore, is undesirable in a network analyzer requiring frequent switching between multiple frequency ranges. Electronic switches, on the other hand, have unlimited switching life, but have large insertion losses, e.g. 7-8 dB, at higher frequencies, e.g. 40 GHz. Since the output power of a signal generator at such high frequencies is typically quite low, the use of electronic switches at such frequencies is also undesirable.